Given a string and a replacement character, replace all vowels (a, e, i, o, u - both uppercase and lowercase) with the replacement character. Return null for null input and empty string for empty input.
Examples:
Input: str = 'hello', replacement = '*'
Output: 'h*ll*'
Explanation: e and o are vowels, replaced with *
Input: str = 'APPLE', replacement = '#'
Output: '#PPL#'
Explanation: A and E are uppercase vowels, replaced with #
Input: str = 'gym', replacement = '-'
Output: 'gym'
Explanation: No vowels found, string unchanged
Input: str = null, replacement = '*'
Output: null
Explanation: Null input returns null
Input: str = '', replacement = '*'
Output: ''
Explanation: Empty string returns empty string
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